style: BuildFlow.slnx → build-flow.slnx（文件名小写）

- 解决方案文件名改为小写
- 架构测试 FindRepoRoot 同步改文件名查找

tests/BuildFlow.Architecture.Tests/HostArchitectureTests.cs

@@ -77,7 +77,7 @@ public class HostArchitectureTests
         var current = new DirectoryInfo(AppContext.BaseDirectory);
         while (current is not null)
         {
-            if (File.Exists(Path.Combine(current.FullName, "BuildFlow.slnx")))
+            if (File.Exists(Path.Combine(current.FullName, "build-flow.slnx")))
             {
                 return current;
             }
@@ -85,7 +85,7 @@ public class HostArchitectureTests
             current = current.Parent;
         }
 
-        throw new InvalidOperationException("无法定位 BuildFlow.slnx，架构测试无法确定仓库边界。");
+        throw new InvalidOperationException("无法定位 build-flow.slnx，架构测试无法确定仓库边界。");
     }
 
     private static bool ContainsPathSegment(string path, string segment)